Mechanisms for Radiation Damage to DNA: LET Effects

描述（申请人提供）：我们的实验室继续开发DNA辐射诱导损伤的综合模型，该模型描述了从初始轨道结构和DNA离子-自由基-激发态通过空穴和电子转移形成的物理化学事件，到涉及导致二次自由基的自由基过程的化学事件，最后，涉及导致DNA损伤如碱基和糖损伤、链断裂和相关碱基释放的结合和氧化还原过程。损伤部位的空间分布是我们模型的关键部分，因为它决定了辐射损伤的生物学效应。目前提出的努力的总体目标是测试这个整体模型的几个方面，适当地修改它，从而阐明不同的线性能量转移（LET）的辐射对DNA的辐射损伤的基本机制。这些研究将在强调辐射直接影响的条件下进行，并将采用磁共振光谱法（ESR）、产物分析、伽马和回旋加速器重离子束辐照以及包括时间相关密度泛函理论（TD-DFT）在内的理论建模。第一个目标是检验空穴从糖-磷酸骨架到DNA碱基的转移过程的机制是碱基序列和温度依赖性的假设。选择用于研究的寡聚体序列将提供从DNA骨架通过A运行到DNA中的远程G的远程空穴转移的机制的序列依赖性和温度依赖性。第二个目标将测试的假设，即在DNA糖磷酸骨架的C5'和C3'位点的两个链断裂自由基的产量是LET依赖性的，并分别由特定的氧化和还原机制产生。我们将检验这样的假设，即随着LET的增加，这些自由基的空间聚集增加，形成多个邻近的链断裂（多个损伤位点）;然而，糖自由基的最高产量并不恰好发生在离子束路径的布拉格峰处，这表明离子和自由基的重组抑制了轨道末端的簇形成。第三个目标将采用理论计算来进一步测试和确认每个目标中提出的分子机制。特别重要的将是使用DFT理论准确地预测核心激发态和测试的假设，一个低能量的电子可以诱导双链断裂，并进一步的糖位点的双氧化导致链断裂没有自由基的参与。我们相信这些努力将使我们能够 建立对生物医学研究重要的基本辐射过程的新见解。

DESCRIPTION (provided by applicant): Our laboratory continues the development of a comprehensive model of DNA radiation induced to damage, one that describes physicochemical events from the initial track structure and DNA ion-radical-excited state formation through hole and electron transfer, to chemical events involving free radical processes that lead to secondary radicals and, finally, to combination and redox processes that result in DNA damage such as base and sugar damage, strand scission and associated base release. The spatial distribution of the damage sites is a critical portion of our model as it determines the biological effectivenes of the radiation damage. The overall goal of the current proposed effort is to test several aspects this overall model, to modify it as appropriate and thereby elucidate fundamental mechanisms of radiation damage to DNA by radiations of varying linear energy transfer (LET). These studies will be performed under conditions that emphasize the direct effect of radiation, and will employ magnetic resonance spectroscopies (ESR), product analyses, gamma and cyclotron heavy ion-beam irradiation, as well as theoretical modeling including time dependent density functional theory (TD- DFT). The first aim will test the hypotheses that the mechanism of hole transfer process from the sugar- phosphate backbone to the DNA bases is base sequence and temperature dependent. Oligomer sequences chosen for study will provide the sequence dependence and temperature dependence of the mechanism of long range hole transfer from the DNA backbone through A runs to a remote G in DNA. The second aim will test the hypothesis that the yields of two strand break radicals, at C5' and C3' sites in the DNA sugar phosphate backbone are LET dependent and are produced by specific oxidative and reductive mechanisms, respectively. We will test the hypothesis that, as LET increases, spatial clustering of these radicals increases to form multiple proximate strand breaks (multiple damage sites); however, the highest yields of sugar radicals do not occur precisely at the Bragg peak of the ion beam path. This would suggest that ion and radical recombinations inhibits cluster formation at the track ends. The third aim will employ theoretical calculations to further test and confirm molecular mechanisms proposed in each of the aims. Especially significant will be the use of DFT theory to accurately predict core-excited states and test the hypotheses that one low energy electron can induce a double strand break and further that double oxidation of sugar sites leads to strand breaks without radical involvement. We believe these efforts will allow us to establish new insights into fundamental radiation processes important for biomedical research.